The present invention relates to a method of measuring level of filling by means of a hot-wire measuring device in which a hot-wire transmitter is charged with a predetermined constant current, and an increase in the voltage over the hot-wire transmitter is detected within a predetermined period of time.
Such hot-wire transmitters are used, in particular, in motor vehicles for measuring various filling levels, such as, for instance, the oil level in the engine of a motor vehicle. The current heats the hot wire, the heating being dependent also on the surrounding medium. The course of the heating differs in the case wherein the hot wire or a part thereof is surrounded by air from the case wherein the hot wire is surrounded by a fluid such as oil or gasoline. With increasing resistance, and with constant current, there is a continuously higher voltage drop over the hot-wire transmitter. The increase in voltage, i.e., the difference between the voltage in the unheated hot wire and that in the hot wire, upon heating for a predetermined period of time, then constitutes a measure of the extent to which the hot wire is immersed in the liquid to be measured and, thus, a measure of the filling level of the liquid.
Also in such a method of measuring level it is endeavored to digitally process the results obtained. There is a problem in this connection in that the results obtained frequently contain large inaccuracies. The inaccuracies result, on the one hand, from the fact that the resistances of different hot-wire transmitters vary, and on the other hand from the fact that the voltage increase is very slight as compared with the possibilities of resolution of the analog/digital converters required for a digital evaluation. If the voltage increase, starting from about 3 to 4V, is in the area of about 400 mV, but the analog/digital converter has a reference voltage of only 5V and a resolution of 8 bits, then each increment represents a value of about 19.5 mV. The entire measurement would then be effected with about 20 increments. If, in addition to such rough resolution, there are also uncertainties with respect to the hot-wire transmitter, then the measurement will be too inaccurate for many fields of use.